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1030                           Appendix VI.                                                       j

the cutter circumference. The examples k and / serve as illus-
trations. The worm-hob / is tapered to suit a wheel-cutting
system of Reinecker's own, where the centres of worm and
wheel are never changed during cutting, the worm being simply
advanced along its axis to obtain the depth feed, and thus
the correct shape of the tooth space is obtained right through
the operation.

Lastly, it may be said that the lathe just described will take                    !

work up to 9! ins. diameter and 39 ins.  long, and will  relieve
cutters of from 2 to 40 teeth, having spirals of from 4 to 400 ins.
x                 pitch, right or left hand.     The 'hand' of the spiral is entirely

determined by the change wheel R, Fig. 934, for if the wheel r
turns slowly in one direction the tool lags, but if in the opposite                    T

.         direction it gains, on the work.    Also the number of teeth in                     ;

the blank is determined by the change wheels N, the traverse by                     

those at D, and the depth of cut by a hand advance.                                         \

i|                             P:   197.     Machine    Grinding. The   Emery   grinding

machines on pp. 198-9, it will be observed, have their opera-
tions confined to the accurate sharpening of cutting tools,
especially of milling cutters, twist drills, rimers, and the like. It                     (

was soon found that grinding by emery wheel was an exceedingly                     \

satisfactory  means   of  finishing all  kinds   of  machined work                     j

requiring   extreme   accuracy.     Ordinary   tooled    work   always                     j

required    correction    after   machining   because   of   the    in-                   *k

accuracy produced by clamping or by the pressure of the tool,
and this finishing process was formerly done by hand with file
and scraper in the case of flat surfaces, or by lapping with emery
in that of lathe or bored work, the object being to remove the
metal by means of light cuts with little stress. Lapping con-                     j

sisted of the application of a leather-faced tool or disc of leather                     f

soaked  in oil  and supplied with emery powder.    It is not so                     f

accurate as might be wished because of the tendency to wear
down soft spots more than the hard parts of the work, and
although the principle is still followed for the last finish, a tool of
soft cast iron is preferred, fed with crocus powder and oil. The
old methods would have remained but for the gradual and sure                     '

advance in the perfection of manufacture of the emery wheel.                     *